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CONTENTS
Volume 12, Number 4, April 2012
 

Abstract
In this paper the influence of stiffener location, rise/span ratio and fibre orientation on vibration behavior of corner supported hypar shells is studied by using a four-node hybrid stress finite element. The formulation of the element is based on Hellinger-Reissner variational principle. The element is developed by combining a hybrid plane stress element and a hybrid plate element. Benchmark problems are solved to validate the approach and free vibration response of stiffened orthotropic hypar shells is studied both with respect to fundamental frequency and mode shapes by varying the location of stiffeners, rise/span ratio and fiber orientation.

Key Words
stiffened hypar shell, assumed stress hybrid element, finite element, free vibration

Address
Department of Civil Engineering, Istanbul Technical University 34469, Maslak, Istanbul, Turkey

Abstract
Frequently, steel silos are supported by discrete supports or columns to permit easy access beneath the barrel. In such cases, large loads are transferred to the limited number of supports, causing locally high axial compressive stress concentrations in the shell wall above the supports. If not dealt with properly, these increased stresses will lead to premature failure of the silo due to local instability in the regions above the supports. Local stiffening near the supports is a way to improve the buckling resistance, as material is added in the region of elevated stresses, levelling these out to values found in uniformly supported silos. The aim of a study on the properties of local stiffening will then be to increase the failure load, governed by an interaction of plastic collapse and elastic instability, to that of a discrete supported silo. However, during the course of such a study it was found that, although the failure remains local, the cylinder height is also a parameter that influences the failure mechanism, a fact that is not properly taken into account in current design practice and codes. This paper describes the mechanism behind the effect of the cylinder height on the failure load, which is related to pre-buckling deformations of the shell structure. All results and conclusions are based on geometrically and materially non-linear finite element analyses.

Key Words
cylinder; local supports; elastic buckling; plastic yielding; cylinder height.

Address
Arne Jansseune, Wouter De Corte : Department of Civil Engineering, Faculty of Applied Engineering Sciences, University College Ghent, Ghent University Association, Ghent, Belgium
Wesley Vanlaere, Rudy Van Impe : Department of Structural Engineering, Faculty of Engineering Sciences, Ghent University, Ghent, Belgium

Abstract
This paper presents the mode-shape analysis of the cross-ply laminated composite cylindrical shallow shells. First, the kinematic relations of strains and deformation are given. Then, using Hamilton\'s principle, governing differential equations are developed for a general curved shell. Finally, the stress-strain relation for the laminated, cross-ply composite shells are obtained. By using some simplifications and assuming Fourier series as a displacement field, the governed differential equations are solved by the matrix algebra for shallow shells. Employing the computer algebra system called MATHEMATICA; a computer program has been prepared for the solution. The results obtained by this solution are compared with the results obtained by (ANSYS and SAP2000) programs, in order to verify the accuracy and reliability of the solution presented.

Key Words
structural composites; vibration; anisotropy; shell theory; finite element method (FEM)

Address
Ali Dogan: Department of Civil Engineering, Mustafa Kemal University, 31200 Hatay, Turkey
H. Murat Arslan : Department of Civil Engineering, Cukurova University, 01330 Adana, Turkey

Abstract
Concrete filled steel hollow structural sections (CFSs) are an efficient, sustainable, and attractive option for both ambient temperature and fire resistance design of columns in multi-storey buildings and are becoming increasingly common in modern construction practice around the world. Whilst the design of these sections at ambient temperatures is reasonably well understood, and models to predict the strength and failure modes of these elements at ambient temperatures correlate well with observations from tests, this appears not to be true in the case of fire resistant design. This paper reviews available data from furnace tests on CFS columns and assesses the statistical confidence in available fire resistance design models/approaches used in North America and Europe. This is done using a meta-analysis comparing the available experimental data from large-scale standard fire tests performed around the world against fire resistance predictions from design codes. It is shown that available design approaches carry a very large uncertainty of prediction, suggesting that they fail to properly account for fundamental aspects of the underlying thermal response and/or structural mechanics during fire. Current North American fire resistance design approaches for CFS columns are shown to be considerably less conservative, on average, than those used in Europe.

Key Words
concrete filled steel sections; fire resistance; standard fire testing; structural design.

Address
David Rush, Luke Bisby : BRE Centre for Fire Safety Engineering, University of Edinburgh, UK
Allan Jowsey : International Paint Ltd, AkzoNobel, Newcastle, UK
Athan Melandinos and Barbara Lane : Arup Fire, London, UK

Abstract
The accuracy of the 30% and SRSS rules, commonly used to estimate the combined response of structures, and some related issues, are studied. For complex systems and earthquake loading, the principal components give the maximum seismic response. Both rules underestimate the axial load by about 10% and the COV of the underestimation is about 20%. Both rules overestimate the base shear by about 10%. The uncertainty in the estimation is much larger for axial load than for base shear, and, for axial load, it is much larger for inelastic than for elastic behavior. The effect of individual components may be highly correlated, not only for normal components, but also for totally uncorrelated components. The rules are not always inaccurate for large values of correlation coefficients of the individual effects, and small values of such coefficients are not always related to an accurate estimation of the response. Only for perfectly uncorrelated harmonic excitations and elastic analysis of SDOF systems, the individual effects of the components are uncorrelated and the rules accurately estimate the combined response. In the general case, the level of underestimation or overestimation depends on the degree of correlation of the components, the type of structural system, the response parameter, the location of the structural member and the level of structural deformation. The codes should be more specific regarding the application of these rules. If the percentage rule is used for MDOF systems and earthquake loading, at least a value of 45% should be used for the combination factor.

Key Words
seismic design codes; combination rules; effect of individual components; steel buildings; correlation of effects; MDOF and SDOF systems.

Address
Alfredo Reyes-Salazar*, Federico Valenzuela-Beltran,Eden Bojorquez and Arturo Lopez-Barraza : Facultad de Ingenieria, Universidad Autonoma de Sinaloa, Ciudad Universitaria, Culiacan Sinaloa, Mexico
David de Leon-Escobedo : Facultad de Ingenieria,Universidad Autonoma del Estado de Mexico, Ciudad Universitaria, Toluca, Estado de Mexico, Mexico


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